Tactical high frequency antenna

ABSTRACT

An electrically short (less than lambda /4) antenna for operation in the frequency range from 2 to 30 megahertz (MHz). The antenna system contemplated by the subject invention comprises a short inductively loaded vertical radiating element having a capacitive top termination, a ground system and an automatic remote control of the tuning of the antenna which senses the state of resonance of the antenna and operates to drive a tuning reactance, e.g. a variable capacitor in series with the vertical radiator element to provide the required lambda /4 resonance at a given operating frequency.

United States Patent mi Campbell H5] Sept. 30, 1975 l l TACTILAL HIGHFREQUENCY ANTENNA [75] Inventor: [)(mn Campbell. Etlltttll0\\'lt. NJ.

{7H Assigneei The l'nited States of America as represented In theSecretary of the :\Illl \Nashington. DL',

[22 Filed: .'\la 17, I974 [Ill Appl No. 471L777 [5"] U.S. 343/703;3431M]; 343/745L HlllQ U32; HUIQ Witt 343/703. 750. 745. 752

[Fl] int. (1....

Field of Search [5M References Cited les sns WW7: Felsenheld .UN

Pri'imu'y 1;.\mniriw'Eli Lieberman trmrm'y. .lgmil. m Firni.' l;ithanlidelherg; Robert P.

(iihson; Arthur L. Bouers [57) ABSTRACT An electrically short (less thanM4] antenna for open ation in the frequency range from I to Rt)megahert/ (MHZ). The antenna system contemplated h the subjeet imentioncomprises a short inductively loaded \ertieaI radiating element having acapacitive to termination. at ground system and an automatic remotecontrol of the tuning ot the antenna which senses the state of resonanceof the antenna and operates to drive a tuning reactuncc, eg. a \ariahlecapacitor in series with the \erticnl radiator element to pro\ ide therequired M4 resonance at it goen operating l requency.

14 Claims. 4 Draning Figures TRANSCEIVER Sept. 30,1975

Sheet 1 of 2 3,909,830

US. Patent TACTICAL HIGH FREQUENCY ANTENNA The invention describedherein may be manufactured and used by or for the government forgovernment purposes without payment of any royalty thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates to antennas ofelectromagnetic radiation and more particularly to a trans portableantenna system which is adapted to radiate electromagnetic energyefficiently from a very short radiator compared to the wavelength of theradiated energy.

In military applications, the dimensions of portable ground based orvehicular high frequency antennas are necessarily a small fraction ofthe wavelength. It is well known that ground based antennas which aresubstantially smaller than M4 are usually inefficient electromagneticradiators. In addition the required wide frequency range of interest inthe present invention (2 to 30MHz) imposes serious practical limitationsin tuning and effecting efficient power transfer from the transmitter tothe antenna. For example, known prior art apparatus heretofore utilizeda series whip antenna being in the order of 30 feet in length andgenerally mounted on a plate over the ground or on the side of a vehicleor other supporting means. This type of antenna although being portable,is physically not compatible for certain use, e.g. jungle environmentbecause of the nature of the terrain. Secondly, this type of antenna bynature presents difficulty in being resonated over the desired operatingfrequency range.

One known type of transportable high frequency antenna system attemptingto overcome the inherent difficulties in obtaining an electrically shortantenna for operation in the range from 2 to 3OMH2 is taught for examplein U.S. Pat. No. 3,510,872, .I. H. Mulhaney, which utilizes acollapsible helical antenna element. A second type of electrically shortantenna is disclosed in U.S. Pat. No. 3,209,358, R. A. Felsenheld, whichdiscloses, inter alia, a linear antenna having electronically controlledtuning means. In addition to ferrite core antennas for obtainingcompactness, electrically short radiating antennas have also beenachieved in the prior art by the use of inductively loaded foldedmonopole antennas such as taught in U.S. Pat. No. 3,103,011, E. W.Seeley.

SUMMARY OF THE INVENTION The subject invention is directed to animprovement in high frequency antennas and briefly comprises a shortinductively loaded vertical radiating element hav ing a capacitive toptermination coupled to one end, while the opposite end is coupled to aground system through the series combination of remotely controlledreactance tuning means and energy feeding means. A tuning detectorcoupled to the feeding means is adapted to provide an output signalindicative of the resonant or non-resonant state of the antenna. Thissignal is then coupled to a control unit which is adapted to vary thereactance tuning means until resonance is achieved. When the antennaconsists of a ground based system the ground system consists of acounterpoise comprising a number of radially oriented wires lying in thehorizontal plane and suspended above the ground. In a vehicularconfiguration, the radially oriented wires acting as the counterpoiseare deleted and the grounding system comprises the vehicle body itself.Also, in the ground based configuration, cable choke means areadditionally included in the feeding means for suppressing radiation inthe RF feed lines.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical schematicdiagram illustrative of the land based embodiment of the subjectinvention;

FIG. 2 is a diagram illustrative of a typical plan view of thecapacitive top terminations at one end of the radiating element shown inFIG. 1, as well as the counterpoise consisting of a number of radiallyoriented wires located at the other end;

FIG. 3 is an electrical schematic diagram of a vehicular mountedembodiment of the subject invention; and

FIG. 4 is a partial electrical schematic diagram illustrative of yetanother embodiment of the subject invention.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand more particularly to FIG. 1, the radiating means consists of arelatively short inductively loaded vertical radiator element I0 havinga capacitive top termination 12 connected to the upper extremity thereofand a counterpoise ground system 14 coupled to the other extremity bymeans of the series of combination of a frequency band select inductivereactance 16, a variable fine tuning capacitance 18, and antenna feedpoint means comprising s broadband transformer 20 having its secondarywinding connected between the tuning capacitor 18 and the counterpoise14.

The vertical radiating element 10 comprises a metal mast or wiresupported by plastic or fiber glass tubing and typically having a heighth 6 feet for operation in the high frequency range (HF) of theelectromagnetic spectrum of from 2 to 3OMHz. The top termination 12 iscomprised of a plurality of radially oriented wires or metal tubes 22extending radially outwardly in a horizontal plane from the verticalcentral axis of the radiating element 10. The effect of the capacitivetop termination is to increase radiation efficiency and reduce antennareactance. A means for maintaining the radially oriented elements 22 inthe horizontal plane is one of choice, depending upon the intended use.For example, if the elements 22 are comprised of wires, some form ofinsulating support, not shown, would be required. In vehicularinstallations, the top capacitance elements may also be required to bearranged to reduce wind resistance. In FIG. 2, the top capacitanceelements 22 are shown arranged in a regularly spaced configuration 45apart from one another. This is shown merely for sake of illustrationand is not meant to be interpreted in a limiting sense, however.

Since the physical dimension of the radiating element 10 is only a smallfraction of the wavelength, it acts over a broad frequency range asthough it were a capacitor insofar as its circuit properties areconcerned. Accordingly, a loading inductance I6 is required to achievequarter wavelength (it/4) resonance at a given operating frequency.Since the inductance varies inversely, as the square of the frequency,for a frequency range of 2 to 3OMHZ the inductor I6 must have a rangevariation of 1:225. Accordingly, the inductance 16 is comprised of amulti-tapped inductor having selected portions of the winding adapted tobe shorted out by means of a shorting switch 24.

The operation, the inductive reactance 16 operates substantially as afixed inductance for a selected fre' quency band in a plurality ofoverlapping plurality of in the high frequency (HF) range. Accordingly,the inductor 16 is adapted to be adjusted in steps to provide coarsetuning of the radiating element to a predeter mined frequency bandwithin the HF range. The variable capacitance 18 on the other handadapted to provide a fine tuning adjustment. Thus resonance is obtainedby means of the variable capacitance 18 in series with a substantiallyfixed inductor 16.

the counterpoise I4 is used to reduce ground losses in the ground basedembodiment shown in FIG. 1, and consists of a pluralityof radiallyoriented wires or other type elements 26 disposed in a horizontal planejust above the ground 28. The elements 24 are also shown regularlydisposed in a radial fashion in FIG. 2 for purposes of illustrationonly. Experimentation has shown that in the counterpoise 14 preferablycomprises at least sixteen wire elements 26, each approximately l3 feetlong and suspended 12 inches above the ground 28 in order to effectivelyshield the antenna element and stabilize its resistance to a relativelylow value.

The antenna system resulting from the configuration shown in FIG. 1exhibits a relatively high O which makes remote controlled fine tuningof antenna resonance necessary due to the fact that the antennareactance is sensitive to the presence of nearby objects. Accordingly,the band select inductance value of the inductor 16 is provided by meansof an electrically controlled rotary selector 30 mechanically coupled tothe switch 24 while the variable fine tuning capacitor 18 is controlledby means of an electrically operated reversible motor 32. Both therotary selector 30 and the reversible motor 32 are operated byrespective control signals applied thereto from a control unit 34 withthe motor 32, at least operating in response to an electrical outputsignal from an antenna tuning state detector 36. This will be consideredin greater detail subsequently.

Fine tuning of the antenna is preferably accomplished by means of thevariable capacitor 18 in view of the following considerations. Theantenna tuning range, within a given band, can be calculated from theapproximate formula:

Cum,

Variable vacuum capacitors having the recited values for C and C arecommercially available.

If on the other hand fine tuning by means of variable inductance were tobe used, a relatively narrower tuning range would be achieved in mostbands as exhibited by the formula:

where f and f. are the upper and lower operating frequencies, L and L isthe inductance of the variable inductance utilized as the fine tuningelement, and L denotes the fixed inductance, i.e. inductance of theelement 16 shown in FIG. I. For typical values of L;

IOOuH, L,,,,,, l2p.H and L,,,,-,, [p.H, a ratio of would be obtained,which is substantially less than that obtainable with capacitancetuning. From the foregoing, the ratio (f /fi) utilizing bothcapacitative and inductive fine tuning indicates that fewer bands arerequired to cover the entire HF range (2 to 3OMl-lz) when the variablecapicator tuning method is employed. Obviously. since fewer bands areneeded to cover the frequency range, and since the required capacitortype components are commercially available, capacitor fine tuning ishighly preferable.

In the land based embodiment shown in FIG. 1, an RF sourse such as atransceiver 38 is coupled to the antenna feed point 40 by means of thematching transformer 20 by way of RF transmission line segments 42 and44 and the tuning detector 36. The transmission line 44 is additionallyconfigured to include a cable choke portion 46 which is shown in FIG. 1in block diagrammatic form, but in actuality, consists in having aportion of the transmission line 44 simply formed into a coil. Matchingof the antenna to the feed line by means of the broad band transformer20 is relatively easy, first since resonance is always established atthe operating frequency, the corresponding antenna feed point impedanceis resistive even though for example with 50ohm radio transceivers, thefeed point impedance is usually less than 50ohms when the M4 resonantmode is used. Secondly, in conjunction with the counterpoise 14, theantenna input impedance is relatively constant over a broad frequencyrange, e.g. 2 to l3MHz. At upper frequencies above l3Ml-lz, whenpreferable, the matching transformer 20 may be unnecessary and thereforedeleted. In such a case, the transmission line 42 would be connecteddirectly to the antenna feed point 40 without incurring a substantialmismatch. Alternatively, the transformer 20 could be designed so as toachieve minimum VSWR over the entire HF range from 2 to 3OMHz.

The impedance match between the antenna and transmission line 42 fromthe transceiver 38 is determined by the electrical characteristics ofthe broadband transformer 20 and the tuning state of the antennaradiating element 10. Assuming that the transformer introduces noreactances of its own, a match is obtained only when the antenna isnon-reactive, i.e. in its resonant state. The tuning detector 36 whichfor example preferably comprises a radio frequency phase sensor, wellknown to those skilled in the art, is coupled to the antenna feed pointby means of the broadband matching transformer 20 and the RFtransmission line segment 42. A phase sensor is a device which isadapted to measure the phase relationship between the radiating elementsexcitation voltage and the current and produces a DC output voltagewhich is proportional to the phase difference therebetween. with zerooutput from the detector indicating antenna resonance. An output signalfrom the sensor of a positive polarity for example, indicates that theantenna is in a nonresonant state and has a corresponding reactiveimpedance which is inductive while a negative polarity out put isindicative of a non-resonant state wherein the corresponding reactiveimpedance is capacitive. The phase sensor accordingly must accuratelymonitor the tuning state of the antenna over the entire frequency rangeof from 2 to 3UMHZ.

The DC output signal from the phase sensor 36 is coupled to the controlunit 34 by means of a shielded cable 48 which also includes a portionthereof contigured into a cable choke portion 50 in the same manner asthe cable choke portion 46. The remote control unit contains electricalcircuitry e.g. a servo-system neces sary to effect control of coarse andfine tuning and includes the necessary voltage and power supplies aswell as an antenna tuning state indicator or meter, manual controls anda source of primary power. The DC signal derived from the phase sensor36 for example, is at least used to drive the reversible motor 32mechanically coupled to the variable capacitor 18. When desirable thissignal can also be used to initiate operation of the rotary selector.This is provided by means of a multiwire cable 52. The cable 52 alsoincludes a portion thereof which is configured into a cable chokeportion 54 in the same manner as the cable chokes 46 and 50.

It should be pointed out that when desirable the separate cable chokescould be combined. The purpose of the cable chokes 46, 50 and 54 is tosuppress feed line radiation and reduce ground losses. The cable choke46 in the RF transmission line 44 is the most important and is locatedrelatively close the the antenna for maximum effectiveness and providesa high impedance across its extremities, e.g. Skilohms at frequencieswhere feed line excitation is most troublesome. In the case of thesubject antenna system, feed line excitation appears to be of concern inthe frequency range of from 2 to lOMHz rather than at higherfrequencies. This is explained by the fact that the frequencies aboveapproximately IOMHz, the eounterpoise l4 acts as a better ground. Thecable chokes 46, 50 and 54 moreover become increasingly essential as thedimensions of the eounterpoise 14 decreases.

Referring now to a vehicular installation, attention is directed to theFIG. 3. In such embodiment, the elements are exactly the same as shownand described with respect to the ground based embodiment shown in FlG.l with the exception that the cable chokes 46. 50 and 54 and theeounterpoise l4 illustrated in FIG. 1 are now eliminated. This is due tothe fact that the radiating element is terminated in the vehicle body 56which effectively acts as the eounterpoise. In all other respects, thetwo embodiments are exactly alike.

Another embodiment of the subject invention adapted for either land orvehicular use is shown in FIG. 4. A land based configuration is shownfor purposes of illustration since it discloses utilizing a counterpoise14 as the ground system". Whereas in the earlier embodiments shown inFIGS. 1 and 3, the coarse or band tuning is accomplished by means of amultiple tap single inductor 16 having a selected portion of the windingshorted by means of the rotary selector driven switch 24, the presentembodiment discloses the use of a plurality of inductors 58 selectivelycoupled individually in series with the fine tuning capacitor l8 and thevertical radiator element 10. The selective interconnection is providedby means ofa multiple position two section band selector switchincluding one section 60 which connects one end of the selected inductorto the variable capacitance 18 while the other section 62 connects theopposite end of the same selected inductor to the antenna feed point 40.As in the other embodiments the switch position is mechanicallycontrolled by the roatry selector 30.

Also in the embodiment shown in FIG. 4, the tuning detector 36 islocated between the antenna and the broadband radio frequencytransformer 20. In other words their relative positions are reversedwith respect to the antenna fcedpoint 40 and RF source. ie thetransceiver 38.

The embodiment disclosed in FIG. 4 has several advantages over the twoprevious embodiments in that it is possible in some instances with thisconfigurations to obtain more than one frequency at which resonance isindicated by the phase sensing type tuning detector 36 for a givensetting of the fine tuning capacitor [8 and the band select setting ofthe inductor 16. Although only one of the resonances constitutes thedesired resonance frequency associated with the antenna. the extraneousresonances" results from the relatively complicated capacitive andinductive coupling occuring between the various sections of the coarsetuning (hand select) inductor 16. In principle the extraneous resonancesare not harmful as long as they can be distinguished from the desiredantenna resonance and are somewhat separated in frequency. From apractical standpoint. however. it is desirable and in some appli cationsessential to eliminate all extraneous resonances within the particularband ofoperation in order that the antenna can be resonated withoutambiguity.

By utilizing separate tuning inductors for each selected band within theHF range. only one resonance frequency was observed in each bandcorresponding to the desired antenna resonance. All extraneousresonances occured at frequencies outside the operating band.

Secondly the stray reactances of the transformer 20 can in someinstances cause faulty tuning when the transformer is connected betweenthe phase sensing tuning detector 36 and the antenna. The problem isobviated when the tuning detector 36 is installed between the antennaand the transformer.

The advantage obtained by the configuration shown in FIG. 4 is thatseparate coarse tuning inductors facilitate the selection or setting ofthe various bands. Separatc inductors also yield a lower power loss thana single tapped inductor, and more importantly, since only the desiredantenna resonance is obtained in each band the use of separate inductorsfacilitates automatic tuning.

Under normal circumstances, the antenna radiating element 10 will beinitially in the non-resonant state and thus will be badly matched tothe transmitter portion of the transceiver 38. Consequently, littlepower will be supplied to the non-resonant antenna and the DC errorsignal produced by the phase sensor 36 will be correspondingly small.The polarity of the phase sensor output signal determines of coursewhether to increase or decrease the tuning reactor. As the fine tuningcapacitance 18 is adjusted closer to the value needed to establishantenna resonance, the RF power input to the antenna will increase.correspondingly, the DC output signal of the sensor will also increaseup to a point and then decrease as resonance is approached. Atresonance, the phase sensor output signal from the tuning detector 36will vanish indicating that the feed point current and voltage are inphase. At this point the tuning operation ceases and maximum power isdelivered to the antenna.

The tuning detector 36 up to this point has been described in terms of aphase sensor. There are, however, several limitations associated withthe use of phase information for control of antenna tuning. Onedisadvantage is that the error signal needed to initiate fine tuning isoften very small when the antenna is initially detuned from resonance.Another is that the accuracy of the phase sensor is seriously impairedif the signal emitted by the transmitter has significant harmoniccomponents. While proper operation can nevertheless be ob tained byproper equipment design. another way to indicate antenna resonance is toutilize an indicator which produces a DC output proportional toreflected power. in such a case, reasonance is indicated whereinreflected power is minimized, which is at the point of resonance.However, when the antenna is non' resonant. the reflected powermeasurement method cannot indicate whether the antenna is inductive orcapacitive. Thus for optimum operation, both phase and reflected powersensors would be incorporated in the tuning detector 36 so as toeliminate ambiguities and poor sensitivity.

Furthermore, vehicular installations such as shown by the embodimentillustrated in FIG. 3 requires that the automatic tuning system be incontinuous operation due to the motion ofthe antenna. in baseinstallations, such as shown by the embodiment illustrated in FIG. 1where the antenna is usually undisturbed, the antennas tuning systemmay, when desirable, be deactivated to conserve primary power exceptwhen the operating frequency is changed.

Other modifications and variations of the present invention are possiblein light of the above teachings. For example, the antenna can bemodified to achieve somewhat better electrical efficiency by locatingthe inductor l6 and the variable fine tuning capacitor 18 just beneaththe top capacitance l2, i.e., at the top of the radiating element ratherthan at the bottom. However, such an arrangement would be unattractivefor vehicular use. Also, for installation in airborne vehicles such ashelicopters, the antenna system would utilize the same type of tuningsystem but a modified top capacitance l2 acceptable to air framedesigners would be necessary.

It should be pointed out that the present invention constitutes animprovement over prior art apparatus in a number of respects. Forexample, the top termination capacitance reduces antenna height andyields higher efficiency than ifa single wire of the same height wereused. Moreover, the tuning system covers the entire high frequency range(2 to MH2) without complexity and lends itself to manual or automaticcontrol. The tuning system, moreover, provides more efficient powertransfer to the antenna than is available with present commercialcouplers. Only two high Q reactors are used, one of which comprises avariable capacitor in place of a variable inductor commonly used hereto-LII fore but which requires sliding or rolling contacts which are asource of erratic operation, noise and power loss, For the ground basedinstallation, the isolated counterpoise in combination with the cablechoke effectively shields the antenna against strong variations in theground characteristics and decreases power losses. Even though itsdimensions are modest, the counterpoise is usually a more efficientground system than one of Comparable size composed of wire radials lyingon the earth's surface or buried. in addition, there is theoreticalevidence indicating that up to 98% of the total ground loss occurswithin a radial distance of 0.05)\ from the antenna radiator axis in thelow HF range which provides justification for the use of thecounterpoise as set forth in the present specification.

Accordingly, having thus set forth what is at present considered to bethe preferred embodiments ofthe subject invention,

I claim:

1. A tunable high frequency antenna system, typically but not restrictedfor use in the frequency range of from 2 to 30MH2, comprising incombination:

an electrically short, normally vertical radiating element having aphysical length less than a quarter wavelength for the desired frequencyof operation;

a capacitive top termination coupled to the upper extremity of saidradiating element;

a first controlled electrical reactance connected in series to saidradiating element comprising an inductor having a plurality ofinductance taps, said first reactance being selectively adjustable invalve to provide a coarse tuning of said radiating element to apredetermined frequency band within a selected frequency range comprisedof a plurality of overlapping bands;

a switch means coupled to said inductance taps for selectivelyconnecting a portion of said inductance with said radiating element;

a second controlled electrical reactance connected in series with saidradiating element and said first reactance wherein said second reactancecomprises a variable capacitance to provide a fine tuning of saidradiating element within said predetermined frequency band;

first conduit means for feeding a radio frequency signal to saidradiating element including a broadband transformer having a secondarywinding connected in series with said radiating element and said firstand second reactances;

a ground system connected in series with said first and secondreactances, and said secondary winding;

second circuit means coupled to said first circuit means and beingresponsive to the resonance state of said radiating element to providean electrical output signal indicative of said resonant state; and

third circuit means coupled to said second circuit means and beingresponsive to said output signal provided thereby to generate a controlsignal to vary said first and second reactances in directions to effectresonance of said radiating element,

means controlled from said third circuit means for remotely operatingsaid switch means coupled to said inductance taps for selectivelyshortening out a selected portion of said inductor.

2. The antenna system as defined by claim 1 and additionally includingelectrically operable driver means responsive to said control signal formechanically varying said capacitance.

3. The antenna system as defined by claim 1 wherein said ground systemcomprises a counterpoise located a predetermined distance above theground.

4. The antenna system as defined by claim 3 wherein said counterpoisecomprises a plurality of radially oriented wire conductors lyingsubstantially in a horizontal plane and being suspended a predetermineddistance above the ground.

5. The antenna system as defined by claim 3 wherein said first circuitmeans includes an RF transmission line, a portion of which comprises acable choke for op erating in combination with said counterpoise tosuppress feed line current and ground losses.

6. The antenna system as defined by claim 1 wherein said first circuitmeans includes an RF transmission line and a primary winding coupled tosaid secondary winding and said primary winding being coupled to saidradio frequency signal via said transmission line; and wherein saidsecond circuit means comprises a tuning detector coupled to said primarywinding of said matching transformer.

7. The antenna system as defined by claim I wherein said first circuitmeans includes:

an RF transmission line, a primary winding coupled to said secondarywinding and being coupled to said radio frequency signal by means ofsaid trans mission line; and

wherein said second circuit means comprises a tuning detector includingcircuit means for being coupled between the secondary winding and saidradiating element.

8. The antenna system as defined by claim 7 wherein said ground systemcomprises a counterpoise located a predetermined distance above theground; and

wherein said RF transmission line includes a cable choke portion thereinfor operating in combination with said counterpoise to suppress feedline current and ground losses.

9. The antenna system as defined by claim 8 wherein said second conduitcircuit means and said third circuit means are coupled together by meansof a transmission line a portion of which is configured as a cablechoke; and

wherein said third circuit means and said second reactance is coupled bymeans of an electrical cable a portion of which comprises a cable choke.

10. The antenna system as defined by claim I wherein said ground systemcomprises a vehicle body.

11. The antenna tuning system as defined by claim l wherein said secondcircuit means comprises phase sensor apparatus adapted to measure thephase rela tionship between the excitation voltage and current of saidradio frequency signal.

12. The antenna system as defined by claim 1 wherein said capacitive toptermination comprises a plurality of elements extending radially outwardfrom said radiating element in a plane substantially parallel to saidground system.

13. The antenna system as defined by claim 3 further including anelectrically driven rotary selector switch coupled to said taps andbeing operable to short a selected portion of said inductor in responseto another control signal from said third circuit means to effect coarsetuning of said radiating element.

14. The antenna system as defined by claim 13 wherein said secondcircuit means and said third circuit means are coupled together by meansof an electrical transmission line, a portion of which is configured asa cable choke; and

wherein said third circuit means and said second reactance is coupled bymeans of an electrical cable,

a portion of which comprises a cable choke.

1. A tunable high frequency antenna system, typically but not restrictedfor use in the frequency range of from 2 to 30MHz, comprising incombination: an electrically short, normally vertical radiating elementhaving a physical length less than a quarter wavelength for the desiredfrequency of operation; a capacitive top termination coupled to theupper extremity of said radiating element; a first controlled electricalreactance connected in series to said radiating element comprising aninductor having a plurality of inductance taps, said first reactancebeing selectively adjustable in valve to provide a coarse tuning of saidradiating element to a predetermined frequency band within a selectedfrequency range comprised of a plurality of overlapping bands; a switchmeans coupled to said inductance taps for selectively connecting aportion of said inductance with said radiating element; a secondcontrolled electrical reactance connected in series with said radiatingelement and said first reactance wherein said second reactance comprisesa variable capacitance to provide a fine tuning of said radiatingelement within said predetermined frequency band; first conduit meansfor feeding a radio frequency signal to said radiating element includinga broadband transformer having a secondary winding connected in serieswith said radiating element and said first and second reactances; aground system connected in series with said first and second reactances,and said secondary winding; second circuit means coupled to said firstcircuit means and being responsive to the resonance state of saidradiating element to provide an elecTrical output signal indicative ofsaid resonant state; and third circuit means coupled to said secondcircuit means and being responsive to said output signal providedthereby to generate a control signal to vary said first and secondreactances in directions to effect resonance of said radiating element;means controlled from said third circuit means for remotely operatingsaid switch means coupled to said inductance taps for selectivelyshortening out a selected portion of said inductor.
 2. The antennasystem as defined by claim 1 and additionally including electricallyoperable driver means responsive to said control signal for mechanicallyvarying said capacitance.
 3. The antenna system as defined by claim 1wherein said ground system comprises a counterpoise located apredetermined distance above the ground.
 4. The antenna system asdefined by claim 3 wherein said counterpoise comprises a plurality ofradially oriented wire conductors lying substantially in a horizontalplane and being suspended a predetermined distance above the ground. 5.The antenna system as defined by claim 3 wherein said first circuitmeans includes an RF transmission line, a portion of which comprises acable choke for operating in combination with said counterpoise tosuppress feed line current and ground losses.
 6. The antenna system asdefined by claim 1 wherein said first circuit means includes an RFtransmission line and a primary winding coupled to said secondarywinding and said primary winding being coupled to said radio frequencysignal via said transmission line; and wherein said second circuit meanscomprises a tuning detector coupled to said primary winding of saidmatching transformer.
 7. The antenna system as defined by claim 1wherein said first circuit means includes: an RF transmission line, aprimary winding coupled to said secondary winding and being coupled tosaid radio frequency signal by means of said transmission line; andwherein said second circuit means comprises a tuning detector includingcircuit means for being coupled between the secondary winding and saidradiating element.
 8. The antenna system as defined by claim 7 whereinsaid ground system comprises a counterpoise located a predetermineddistance above the ground; and wherein said RF transmission lineincludes a cable choke portion therein for operating in combination withsaid counterpoise to suppress feed line current and ground losses. 9.The antenna system as defined by claim 8 wherein said second conduitcircuit means and said third circuit means are coupled together by meansof a transmission line a portion of which is configured as a cablechoke; and wherein said third circuit means and said second reactance iscoupled by means of an electrical cable, a portion of which comprises acable choke.
 10. The antenna system as defined by claim 1 wherein saidground system comprises a vehicle body.
 11. The antenna tuning system asdefined by claim 1 wherein said second circuit means comprises phasesensor apparatus adapted to measure the phase relationship between theexcitation voltage and current of said radio frequency signal.
 12. Theantenna system as defined by claim 1 wherein said capacitive toptermination comprises a plurality of elements extending radially outwardfrom said radiating element in a plane substantially parallel to saidground system.
 13. The antenna system as defined by claim 3 furtherincluding an electrically driven rotary selector switch coupled to saidtaps and being operable to short a selected portion of said inductor inresponse to another control signal from said third circuit means toeffect coarse tuning of said radiating element.
 14. The antenna systemas defined by claim 13 wherein said second circuit means and said thirdcircuit means are coupled together by means of an electricaltransmission line, a portion of which is configured as a cable choke;and wherein saiD third circuit means and said second reactance iscoupled by means of an electrical cable, a portion of which comprises acable choke.